1. Field of the Invention
The present invention relates generally to surface mount packages for integrated circuits and more particularly to a surface mount package having a U-shaped via extending around the transmission via of the package substrate to allow the integrated circuit to be operated at higher microwave frequencies.
2. Description of the Prior Art
Surface mount packages, such as shown in Seieroe et al., U.S. Pat. No. 5,117,068, are readily known in the art. In general the purpose of such packages is to house integrated circuits in an environmentally protected enclosure, while at the same time providing electrical connections from the external package leads to the internal enclosed integrated circuit. As illustrated in FIGS. 1-4, a typical prior art surface mount package includes a lead-frame 2 which preferably matches the lead-frame of a conventional TO-8 package and includes outwardly extending leads 14 such as input, output and bias leads. A substrate 4, preferably of alumina, includes solid metal vias 6 extending therethrough which electrically connect the lead-frame 2 to an integrated circuit 8 mounted in the package. The lead-frame 2 and a seal-ring 10 are brazed to each face of the alumina base 4 in a brazing operation. After inserting the integrated circuit into the package, lid 12 is sealed to seal-ring 10 to complete the package.
The integrated circuit 8 is connected to the internal floor of the package by means of conductive epoxy or metal attach 16. Ground vias 18 are provided to electrically connect the internal ground plane 16 to the lead frame 2. Internal package leads 20, which are fabricated as part of the surface mount package make the electrical connections from the integrated circuit 8 to the lead vias 6. The metal filled vias 6 are connected to the external leads 14 during a braze operation.
As the AC frequency of the signal being sent to the IC through lead vias 6 increases beyond about 8 GHz in prior art surface mount packages, it has been found that the electrical losses and interference in the package increase beyond acceptable limits. This is so because, in microwave applications, the transmission medium and device interconnections introduce inductance and capacitance which degrade system performance. Accordingly, it is desirable to have a surface mount package suitable for frequencies beyond 8 GHz in which high frequency devices, such as cellular radio, may operate. Furthermore, it is desirable to have a method for increasing the usable frequency range of other types of integrated circuits.